The present invention generally relates to metallization of optical fibers, and more particularly, to an improved method for the metallization of an optical fiber using a combination of deposition by evaporation and immersion in an electroless solution,
An optical fiber is a filament of transparent dielectric material, usually glass or plastic, and usually circular in cross section, that guides light. An optical fiber usually has a cylindrical core surrounded by, and in intimate contact with, a cladding of similar geometry. Cladding of optical fiber is a layer of material of lower refractive index, in intimate contact with the core material of higher refractive index. The refractive index of a medium is the ratio of the velocity of propagation of an electromagnetic wave in a vacuum to its velocity in the medium,
Fiber-optic technology has many applications requiring the formation of a rigid, high-reliability bond to optical fibers at their ends or at mid-section locations. Bonding or soldering may be facilitated by the metallization of the optical fiber, thus providing a convenient and efficient material to which other components can be soldered using low-temperature solders. It is important that this metallization will not adversely affect the glass fiber or its optical or mechanical properties, as well as the level of stress in the coatings, while providing good connectivity and good conductivity.
A description of the prior art is now presented by reference to Filas, R. W., xe2x80x9cElectroless Metallization of Silica Optical Fiber for Hermetic Packagingxe2x80x9d in ASME Advances in Electronic Packaging, v. 1 (1997). In general, aqueous metal deposition and vacuum deposition are the processes of choice for metallization since these do not adversely affect the low heat tolerance of the polymer coatings usually applied to the glass fibers. The aqueous metal deposition is either based on electroplating or on an electroless process. The vacuum deposition is either based on sputtering or on evaporation. The latter is preferred since sputtering may introduce mechanical damage into the delicate fiber. A critical and comprehensive review of the various deposition processes is reported by Filas, R. W., xe2x80x9cMetallization of Silica optical Fibersxe2x80x9d in MRS Symposium Proc., v. 531 (1998). In U.S. Pat. No. 5,959,595 (1999), Martin et al, disclose a conductive material having a core contacting surface coated with an adhesive material.
The key to the aqueous metal deposition process is a sensitization step using a stannous fluoride solution. Subsequent treatment includes immersion in a palladium chloride/HCl solution and commercially available electroless nickel and displacement gold solutions. U.S. Pat. No. 5,380,559 to Filas, et al, (1995) describes the electroless metallization of an optical fiber for hermetic packaging.
The prior art literature on the subject of electroless metallization describes the basic process of electroless deposition. The most common metals used are copper and nickel. The overall process comprises the steps of bare fiber surface preparation (such as stripping or cleaning), sensitization, activation, and electroless metal deposition. The main disadvantage is a use of sensitization and activation steps which provide adequate adhesion of metal layers only within a very narrow process window and hence, the process is problematic and adhesion may fail.
Strong adhesion can be provided by the use of vacuum evaporation technique, however, the main disadvantage of the evaporation process for fiber metallization is that the evaporated layers get highly stressed for sufficiently thick metal layers. So, it sets a limit to the metallized layers in terms of thickness. For example, evaporated nickel becomes stressed above a thickness of several thousands angstroms. The stressed nickel eventually cracks and peels-off. Thus, only thin metal layers can be deposited onto optical fiber by evaporation.
The main disadvantage of electroplating is that electroplating requires electrical contacts from the fiber to the power source, which also complicates the process. Furthermore, there is still the need to activate the surface before electroplating as in electroless process.
It would be therefore desirable to have a metallization process that provides an efficient method for producing metal coating with strong adhesion to glass fiber, low stresses in the metallization layer and, at the same time, adequate thickness for subsequent soldering/welding and other technological procedures.
Accordingly, it is a principal object of the present invention to overcome the disadvantages associated with prior art metallization processes, by providing an improved method of producing high-quality metallized optical fiber with the required length at the required location along the fiber, which is able to be soldered and connected to mechanical components while reducing the level of stress in the coatings (metal on glass) and providing both good adhesion, conductivity and connectivity.
Because there is no single method answering all these requirements, the present invention combines two metallization techniques: vacuum evaporation without heating of the fiber, and electroless deposition. A thin metal layer, comprising an adhesion layer and a seed layer, is deposited by vacuum evaporation, providing superior adhesion and serving as a seed for the subsequent electroless deposition. The electroless layer provides the flexibility to apply any required metal thickness with low or negligible stress.
In accordance with a preferred method of the present invention, there is provided a method for the metallization of optical fibers, the method comprising the steps of;
evaporating a thin metal layer on the optical fiber, comprising an adhesion layer promoting adhesion to the optical fiber, and a seed layer; and
applying metal to the seed layer by electroless deposition.
The inventive combination of evaporation and electroless processes allows the resulting adequately thick metallized layers to be low stress, or even stress free as well as strongly adhesive. The type of metals, metallization length, location along the fiber, as well as metallization thickness may vary between alternative applications.
Other features and advantages of the invention will become apparent from the following drawings and description.